962 THE VASCULAE SYSTEM. 



Each posterior intercostal vein is provided with valves, both at its termination and 

 along its course, which prevent the blood flowing towards the anterior aspect of the 

 thoracic wall. Its tributaries are derived from the adjacent muscles and bones, and a 

 short distance from its termination it receives a posterior tributary which passes to it 

 between the transverse processes of the vertebrae. This posterior vessel is formed by the 

 union of small veins which issue from the muscles of the back, from the anterior and 

 posterior spinal plexuses which lie respectively in front of the bodies and behind the 

 arches of the vertebrae, and by venous channels which issue through the intervertebral 

 foramina ; the latter vessels commence in the vertebral canal, where they are connected with 

 the anterior and posterior spinal veins. 



VEN^E ANONYMS. 



The innominate veins (Figs. 756 and 757) are two in number, right and left. 

 They return blood from the head and neck, the upper extremities, the upper part, 

 of the posterior wall of the thorax, the anterior wall of the thorax, and the upper 

 part of the anterior wall of the abdomen. Each innominate vein commences 

 behind the medial end of the clavicle of the corresponding side, and is formed by 

 the union of the internal jugular and subclavian veins ; the two innominate veins 

 terminate by uniting together, at the lower border of the first costal cartilage of 

 the right side, to form the superior vena cava. To reach that point the left vein 

 has to pass from left to right behind the manubrium sterni, and it is therefore about 

 three times as long as the right vein. The innominate veins do not possess valves. 



The right innominate vein is a little more than 25 mm. (1 inch) in length. 

 It descends almost vertically to the lower border of the first costal cartilage, and 

 terminates in the superior vena cava. 



Relations. It is in relation, anteriorly, with the medial end of the clavicle and the 

 sterno-hyoid and sterno-thyreoid muscles. It partly overlaps the innominate artery, which 

 lies to its left side, and it is in front of the internal mammary artery, the right vagus 

 nerve, and the upper end of the right pleural sac. The phrenic nerve and the accompany- 

 ing vessels run along its right side, and intervene between it and the right pleural sac. 



Tributaries. In addition to the veins by the union of which it is formed, the right 

 innominate vein receives the right vertebral and internal mammary veins, the first right 

 posterior intercostal vein, and sometimes the right inferior thyreoid vein. The right 

 lymphatic duct also opens into it. 



The left innominate vein passes from left to right, with a slight obliquity 

 downwards, behind the upper part of the manubrium sterni, to the lower border 

 of the first right costal cartilage, where it terminates in the superior vena cava. 

 It is about 60 to 75 mm. (3 inches) long. 



Relations. It is covered anteriorly, in the greater part of its extent, by the sterno- 

 hyoid and sterno-thyreoid muscles, but at its right extremity it is slightly overlapped by 

 the right pleura, and in the median plane the remains of the thymus intervene between it 

 and the posterior surface of the sternum. It rests, posteriorly, upon the left pleura, the 

 left internal mammary artery, the left subclavian artery, the left phrenic, and the left vagus 

 nerves, the left superior cardiac branch of the sympathetic, the inferior cervical branch 

 of the left vagus, the left common carotid artery, the trachea, and the innominate artery. 



Its lower border is in relation with the arch of the aorta, and on its upper border it 

 receives the inferior thyreoid vein of one or both sides. 



Tributaries. It receives the vertebral, internal mammary, inferior thyreoid, superior 

 intercostal veins of its own side, the first left posterior intercostal vein, and some peri- 

 cardial, thymic, anterior bronchial, and anterior mediastinal veins. Sometimes the right 

 inferior thyreoid vein joins it, but not uncommonly that vessel terminates in the right 

 innominate vein or in the commencement of the superior vena cava. 



The thoracic duct opens into it just at the angle of junction of the internal jugular 

 and subclavian veins. 



Venae Mammariae Internae The Internal Mammary Veins. Each internal 

 mammary artery is accompanied by vense comites ; they commence by the union of 

 the vene comites of the superior epigastric and musculo-phrenic arteries, between the 

 sixth costal cartilage and the trans versus thoracis ; and at the upper part of the thorax 



